Few insulation products generate more controversy in the UK construction industry than multifoil reflective insulation. Manufacturers have historically claimed equivalence to 200 mm or more of mineral wool from a product only 25–40 mm thick. Building control bodies, the BRE and independent test houses have repeatedly pushed back on those claims. The following is an attempt to set out what multifoil actually does, where the physics supports its use, and where it falls short.
How Multifoil Works
Multifoil products consist of multiple layers of low-emissivity (low-e) aluminium foil separated by thin wadding (polyester, bubble wrap, or foam spacers). The insulating effect comes from two mechanisms:
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Radiation reflection: low-e aluminium surfaces have an emissivity of 0.03–0.05 compared to 0.9 for most building materials. A low-e surface reflects approximately 95–97% of incident long-wave (thermal) radiation rather than absorbing and re-emitting it.
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Enclosed air layer resistance: each layer of foil with an adjacent air gap adds a small resistance to conductive and convective heat transfer.
The total thermal resistance (R-value) depends on the number of layers, the thickness of each air gap, and critically, the conditions on either side of the installed product. This is where the controversy lies. Reflective insulation only performs as claimed if there are still-air gaps (minimum 20–25 mm) on both sides of the product, those gaps are sealed from convection, and the surfaces facing the air gap are actually low-emissivity (not contaminated by dust or moisture, which raises emissivity toward conventional materials).
Claimed vs Tested Performance
The performance claims that appear in marketing literature are typically derived from BM TRADA or similar test-house assessments under idealised laboratory conditions: perfect installation, clean foil surfaces, well-sealed air gaps, no air movement. Under these conditions a good multifoil product achieves R-values of approximately 1.5–2.5 m²K/W, which is equivalent to 65–110 mm of glass wool (λ 0.044).
Independent and in-situ studies give lower figures:
- BRE guidance (BR 443:2019, Conventions for U-value Calculations) allows assessed equivalent thickness methodology for reflective layers but flags that tested performance typically falls in the 40–80 mm mineral wool equivalent range for complete multifoil systems.
- A 2014 DECC-commissioned study of in-situ performance of reflective insulation products found average U-values significantly worse than claimed, with some installations performing no better than the uninsulated baseline.
- Building control officers and SAP assessors routinely accept only BBA certificate values or specific BRE test results rather than manufacturer-published R-values.
The core issue is installation sensitivity. In a real roof construction, air gaps are rarely sealed against convection, foil surfaces pick up dust within months, and compression during installation collapses the critical air spaces.
Required Air Gaps and Installation Details
For a multifoil product to perform anywhere near its stated R-value, the following conditions must be met:
- Air gap on cold side (outer): minimum 25 mm between multifoil and roof deck or sarking board. This is typically achieved via counter-battens.
- Air gap on warm side (inner): minimum 25 mm between multifoil and internal ceiling or plasterboard. Achieved via service battens or studwork.
- Edges sealed: the perimeter of each air gap cavity must be sealed to prevent convective bypass; open-edged cavities allow warm air to circulate and dramatically reduce effective resistance.
- Foil surface condition: clean, uncompressed, facing into the cavity.
These requirements add installation complexity and material cost (counter-battens, service void). In many retrofit applications, the installation quality required is not achieved, and the product performs substantially below its stated value.
Where Multifoil Provides Genuine Value
There are applications where multifoil products offer a legitimate performance contribution:
Thin roof retrofits: where rafter depth is only 75–100 mm and there is no way to achieve adequate mineral wool depth without losing ceiling height, a multifoil over the rafter deck (under a ventilated counter-batten arrangement) combined with mineral wool in the rafter bays can contribute meaningfully to achieving 0.18–0.22 W/m²K without major structural alteration.
Agricultural and industrial buildings: non-habitable structures where Part L targets do not apply and condensation control is the primary concern (multifoils act as combined vapour control and partial insulation layer), multifoils are well-established and cost-effective.
Summer shading in lightweight construction: in Permitted Development garden buildings, home offices and similar, reflective foils reduce solar heat gain significantly in summer — a genuine and uncontested benefit that complements rather than replaces insulation.
Under floor (suspended timber): a multifoil stapled to the underside of joists with adequate air gap above and below can contribute 0.5–1.0 m²K/W while providing some draught resistance.
Where It Doesn’t Work
Multifoil cannot substitute for conventional insulation in the following scenarios:
- Cold loft floors: the installation conditions required (sealed air gaps, clean foil) are not achievable on a horizontal surface with foot traffic above
- Cavity walls: no air gap can be maintained; the product functions purely as a thin conductive layer and adds negligible resistance
- Under solid floors: no air gap; no radiant mechanism
- As the sole insulation in a Part L compliant new build: building control will require evidence of U-value compliance, and multifoil-only systems cannot reliably achieve 0.11–0.18 W/m²K in habitable roof constructions without substantial supplementary insulation
Cost and Comparison
| Product | Supply cost (£/m²) | Realistic R-value (m²K/W) | Effective mineral wool equiv. (mm) |
|---|---|---|---|
| Basic bubble/foil (1 layer) | £2–£4 | 0.2–0.5 | 10–20 mm |
| Standard multifoil (5–8 layers) | £5–£9 | 0.8–1.5 | 35–65 mm |
| Premium multifoil (10–12 layers) | £9–£14 | 1.3–2.2 | 55–95 mm |
| 100 mm glass wool roll | £3–£5 | ~2.3 | 100 mm |
| 100 mm rock wool slab | £4–£7 | ~2.5 | 100 mm |
On a cost-per-unit-of-thermal-resistance basis, mineral wool rolls outperform multifoil products in most applications. The multifoil premium makes sense only when space is the primary constraint and installation conditions can be rigorously controlled.
Building Regulations and SAP Compliance
Multifoil products assessed under BR 443 and holding a current BBA certificate or equivalent third-party certification can be used in SAP calculations, but the following conditions apply:
- The assessed equivalent thickness or R-value from the BBA certificate (not the manufacturer’s datasheet) must be used
- Air gap conditions specified in the BBA certificate must be demonstrated in the construction drawings
- Building control has discretion to require additional independent evidence or in-situ testing
The BBA has issued certificates for several multifoil products; check that the certificate is current (annual renewal required) and applicable to the specific installation configuration before specifying.
Relevant standards and guidance:
- BR 443:2019 — conventions for U-value calculations, including reflective insulation
- BBA certificates — third-party performance data; check bba.co.uk for specific product certificates
- Approved Document L (2021) — Part L compliance requires verified U-values
- BS 5250:2021 — vapour control and condensation risk (multifoil as VCL element)
- TIMSA Guide — Thermal Insulation Manufacturers and Suppliers Association guidance on reflective insulation assessment
The honest engineering verdict: multifoil products work within their physics — radiant reflection in a clean, sealed air gap is a real phenomenon. The problem is that real-world installations rarely achieve the conditions required for peak performance, and marketing claims have significantly overstated what these products deliver in typical UK construction. Use them as a supplementary layer in space-constrained applications, always combined with conventional insulation, and rely only on BBA-certified R-values.
